Many types of vertebral prostheses have been proposed and patented for implantation in the vertebral disc space after surgical removal of a diseases or damaged disc. Such devices fall into several broad categories. One category includes prostheses advocates the use of pliable synthetic materials in an attempt to mimic the compressibility of the natural human spinal disc. For example, U.S. Pat. No. 5,171,281 (Parsons) discusses a disc spacer which purports to possess mechanical properties akin to those of the normal disc by varying the hardness of the elastomeric material in its nucleus and annulus. U.S. Pat. No. 5,192,326 (Bao) illustrates a prosthetic disc formed from a multiplicity of hydrogel beads having a water content of at least 30%. A semi-permeable membrane covers the beads and is said to permit fluids to flow in and out of the prosthetic nucleus. U.S. Pat. No. 5,071,437 (Steffee) has another approach to a pliable implant. That approach involves upper and lower flat rigid plates sandwiching an elastomeric core made from a polyolefin rubber. U.S. Pat. No. 5,002,576 (Fuhrmann) also discusses a variant on the foregoing approach.
Another approach involves attempts to mimic the shape of a natural spinal disc. U.S. Pat. No. 4,714,469 (Kenna) discusses a spinal implant adapted to replace a disc between adjacent vertebrae using a predetermined thickness and profile to match the space between the vertebrae. The implant includes a porous coating on its surface. U.S. Pat. No. 4,759,766 (Buettner-Janz) illustrates a metallic disc endoprosthesis which has two symmetrical, concave end plates with an intermediate spacing piece. Similarly, U.S. Pat. No. 5,314,477 (Oka) discusses a disc prosthesis including two plates separated by a joint composed of a spherical cap and cylindrical base which attempts to ensure connection between the vertebrae.
Some prostheses emphasize mimicking the shape of the space formed by adjacent vertebral bodies. For example, according to the patentee of U.S. Pat. No. 5,320,644 (Baumgartner), he provides a disc prosthesis having a wedge shape having a wedge angle from the ventral to the dorsal side. The patent says the disc has parallel slits arranged at a right angle to its axis which partly overlap one another, forming leaf springs for transmission of forces from one attachment surface to another. U.S. Pat. No. 5,306,309 (Wagner) provides a spinal disc implant having a right rectangular body including two opposed side faces and two opposed transverse faces. According to the patentees, a convexly curved anterior face defines one end of the right rectangular body, and an outwardly curved face of about that of the anterior surface of a human vertebra.
Each of the foregoing prostheses, however, while addressing some problems, presents others. It is therefore a principal object of the invention to provide a disc prosthesis whose design takes into consideration the anatomy and particularly the geometry of the intervertebral space sought to be filled by the prosthetic device. It is an important and further object to provide a spinal disc prothesis which integrates well with the vertebral bone tissue of adjacent vertebral bodies between which the device is inserted.